The present invention relates generally to the field of ionically conducting separators. The invention particularly relates to methods and processes of fabricating composite membranes especially useful in electrochemical devices requiring a proton conductor such as fuel cells.
The operation of an electrochemical cell requires the occurrence of oxidation and reduction reactions which produce or consume electrons. In operation, an electrochemical cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This mechanism includes one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
One category of electrolytes particularly suitable for use in conjunction with electrochemical cells are proton exchange membranes (PEM). PEMs usually consist of a polymer matrix to which are attached functional groups capable of exchanging cations or anions. The polymer matrix generally consists of an organic polymer such as polystyrene, or other polytetrafluoroethylene (PTFE) analog. In general the material is acid with a sulfonic acid group incorporated into the matrix.
The apparent advantages of using PEMs in fuel cells are numerous. The solid electrolyte membrane is simpler and more compact than other types of electrolytes. Also, the use of a PEM instead of a liquid electrolyte offers several advantages, such as simplified fluid management and elimination of the potential of corrosive liquids. In systems using a PEM, the membrane also serves as an electronically insulating separator between the anode and cathode. However, a number of properties are desirable when using an acid ion exchange membrane as an electrolyte. These include: high ionic conductivity with zero electronic conductivity; low gas permeability; resistance to swelling; minimal water transport; high resistance to dehydration, oxidation, reduction and hydrolysis; a high cation transport number; surface properties allowing easy catalyst bonding, and mechanical strength.
Conventional proton conducting membranes for use in polymer electrolyte membrane (PEM) fuel cells consist of homogeneous polymer films. FIGS. 1 and 2 are schematic diagrams depicting three examples of homogeneous polymer films used in polymer electrolyte membranes. The polymers depicted in FIG. 1 were developed at DuPont and Dow Chemical Company. These polymers represent a class of compounds known as perfluorosulfonic acids (PFSA). These polymers are fully fluorinated, i.e., all of the sites occupied by hydrogen atoms in a conventional hydrocarbon polymer have been replaced by fluorine atoms. This makes the polymers extremely resistant to chemical attack.
PFSA polymers are generally synthesized by the copolymerization of a derivatized, or active, comonomer with tetrafluoroethylene, (TFE), as illustrated in FIG. 3. After synthesis, the thermoplastic polymer, which is both hydrophobic and electrochemically inert, is converted into the active ionomer by a base hydrolysis process, as illustrated. The result of this step is an ionomer in its salt form. This can be converted to the proton form by ion-exchange with a strong acid. The sulfonate functionalities (Rxe2x80x94SO3xe2x88x92) act as the stationary counter charge for the mobile cations (H+, Li+, Na+, etc.) which are generally monovalent.
Another type of polymer, illustrated in FIG. 2, is a derivatized trifluorostyrene (TFS), of the type developed by Ballard. This polymer has a fully fluorinated backbone, but some of the side chains have hydrogen atoms.
The polymer is synthesized by copolymerizing derivatized and non-derivitized trifluorostyrene monomers, as illustrated in FIG. 4. This process also produces an electrochemically inactive thermoplastic. In this system the derivatized monomers create the inert sites while the non-derivatized monomers can be sulfonated, as illustrated in FIG. 4. The result of this process is a proton conducting polymer.
Other homogeneous proton conducting polymers are tabulated in Table I. All of these polymers tend to have poor physical properties making them difficult to handle. For example, sheets of the polymers are easily torn or punctured, thereby requiring a minimum usable thickness of about 2 mils (0.002xe2x80x3, 0.05 mm).
In U.S. Pat. No. 5,547,551 Bahar et. al disclose a composite membrane fabricated by filling the void portion of a porous substantially inert polymer membrane with an ionically conducting polymer. This approach starts with a porous membrane fabricated from an inert polymer, such as polytetrafluoroethylene (PTFE) and converts it to an ion conducting membrane by filling the pores with ionomer deposited from solution. This approach leads to thinner membranes, with membranes less than 1 mil (0.001xe2x80x3, 0.025 mm) produced. These membranes are more conductive than pure PFSA membranes on a conductivity per unit area basis, but have lower specific conductivities. The advantage of these membranes is their strength. A 1 mil membrane produced using this technology is tougher than a conventional 5 mil homogeneous membrane.
In U.S. Pat. No. 5,654,109, Plowman et al. disclose an alternate approach to the fabrication of reinforced membranes. In this approach, a core layer of a tough membrane material is clad with surface layers of highly ionically conductive polymer. Typically all of the layers are PFSA type materials, with the core layer having a significantly higher equivalent weight than the surface layers. Although it would seem that the use of a high equivalent weight polymer would significantly impede the proton flux, it has been experimentally determined that a membrane with a core having an equivalent weight as much as 20% greater than the surface layers exhibits a conductivity equivalent to a solid membrane with the composition of the surface polymer.
While the above methods and processes may allow the fabrication of composite membranes that may present enhanced structural stability and ionic conductivity, the methods used do not allow the flexibility needed in fabricating composite membranes suitable for use in a wide range of applications. Thus there is a great need for membranes and membrane fabricating processes that allow greater flexibility in controlling the physical properties of the composite membranes.
The present invention provides a method of making a composite membrane, comprising: (a) combining a first polymer component with a second polymer component; wherein the first polymer component is a non ion-conducting precursor to an ion-conducting polymer; and (b) converting the first polymer component from the non ion-conducting precursor to the ion-conducting polymer.
The combining of the polymeric components may comprise melting and mixing the polymers, co-polymerizing two or more monomers, co-precipitating a solution of a first polymer and a suspension of a second polymer, filling the pores of a porous polymeric matrix with a solution of a second polymer, or filling the pores of a porous polymeric matrix with a melted polymer.
The invention encompasses a process wherein the step of combining the first and second polymer components comprises mixing a solution of the first component and a suspension of the second component, co-precipitating the first and second polymer components to form a gelatinous mass, and drying the gelatinous mass. The dried gelatinous mass may further be sintered and/or pressed into a sheet, optionally at temperatures of about 150xc2x0 C. or about 300xc2x0 C. Pressing the gelatinous mass into a sheet may be achieved by rolling the mass.
The processes of the invention may also combine the first and second polymer components by forming a solution of the first polymer component, filling the pores of the porous matrix with the solution; removing the solvent from the pores of the matrix to form an essentially pore-free composite material. Alternatively, the first and second polymer components may be combined by melting the first polymer component, filling the pores of the porous matrix with the first polymer component, and cooling the polymer to form an essentially pore-free composite material.
The composite membranes formed by the processes of the invention may comprise regions essentially made of the first polymer component and regions essentially made of the second polymer component. Further, the first polymer component may form an ion-conducting path, generally a hydrophilic path capable of transporting ions through the membrane.
The invention also encompasses a method of making a composite membrane, comprising: (a) initiating a living chain co-polymerization of a first monomer and a second monomer, wherein the first monomer is a non-derivatizable monomer and the second monomer is a non ion-conducting precursor to an ion-conducting monomer; (b) propagating the polymerization to form a co-polymer chain comprising a given number of non-derivatizable monomers and a given number of precursor monomers; (c) fabricating a membrane from the co-polymer chain; and (d) transforming at least a portion of the non ion-conducting precursor monomers into ion-conducting monomers. The non-derivatizable monomers and precursor monomers may be randomly distributed along the co-polymer chain. Depending on the application of the composite membrane, the number of non-derivatizable monomers in the co-polymer chain may be greater or lower than the number of precursor monomers in the co-polymer chain. The process of the invention allows the fabrication of composite membranes wherein the co-polymer chain comprises regions wherein most or all of the monomers are non-derivatizable monomers and regions wherein most or all of the monomers are precursor monomers or ion-conducting monomers obtained by transforming precursor monomers. The regions comprising precursor monomers or ion-conducting monomers obtained by transforming precursor monomers may form an ion-conducting path, generally a hydrophilic path capable of transporting ions through the composite membrane.
Finally, the processes of the invention are applicable to any combination of polymeric components as well as combination of a polymeric component and a non polymeric component. For example, the first component may be a precursor to an ion conducting polymer component selected from long side chain perfluorosulfonic acid, short side chain perfluorosulfonic acid, trifluorostyrene, partially derivatized trifluorostyrene, polystyrene, and mixtures thereof. The second polymer component may be a substantially inert polymer selected from polytetrafluoroethylene (PTFE), perfluoroalkoxy derivative of PTFE (PFA), fluorinated ethylene-propylene copolymer (FEP), polyvinyl chloride, polyvinyl dichloride, polyvinyl fluoride (PVF), polystyrene, polytrifluorostyrene (TFS), polyetherketone, polyethersulfone, polyparaphenylene, and mixtures thereof. When the second component is a porous matrix, the second component may be an inorganic material selected from fiber glass, fibrous quartz, rock wool, fibrous alumina, fibrous silica, or other fibrous silicates or alumino-silicates, and mixtures thereof.